What structural feature in a Sun-like star prevents newly formed helium in the core from mixing with the outer hydrogen layers during the main sequence?

Answer

The radiative zone

Sun-like stars develop a radiative zone surrounding the core where energy moves as photons; this stable layering prevents the processed helium from mixing with the outer layers, which remain pristine hydrogen.

What structural feature in a Sun-like star prevents newly formed helium in the core from mixing with the outer hydrogen layers during the main sequence?
Elementastronomycompositionstargiant